An Inexpensive System for Measuring the Dielectric Properties of Biological Tissues Using an Open-Ended Coaxial Probe

Knowledge of the dielectric parameters of biological tissues is important in a wide range of medical applications of electromagnetic fields. Commercial systems using high-quality vector network analyzers (VNAs) are expensive and can be a barrier to the development of existing and emerging applications. Therefore, the main objective of this work was to investigate the potential of using inexpensive VNAs, which is available on the market, to measure the dielectric parameters of biological tissues. To accomplish this task, a system for the measurement of dielectric properties of biological tissues in the frequency range from 70 MHz to 3 GHz using inexpensive VNAs was developed and presented. The system consists of pocketVNA 2.0 and a custom-made coaxial reflection probe with a diameter of 2.2 mm. The virtual line model of the coaxial probe was used to convert the measured reflection coefficients to the complex permittivity. A total of five ex vivo tissues (heart, muscle, liver, kidney cortex, and fat) and two standard liquids (isopropanol and ethanol) were measured. The lower accuracy and lower signal-to-noise ratio of the inexpensive VNA were compensated for by postprocessing of the measured data, including a time-domain gating and fitting of the evaluated dielectric properties to the Cole-Cole and Debye models for tissues and standard liquids, respectively. The measurement results were compared to the results obtained with a commercial measurement system, and both the standard deviation and mean absolute percentage error (MAPE) were evaluated and presented. The lowest achieved MAPE of biological tissues was for fit relative permittivity 4.35% (fat, $f \lt 200$ MHz) and 0.66% (kidney cortex, $f \gt 200$ MHz) and for fit equivalent electrical conductivity 2.58% (heart, $f \lt 200$ MHz) and 3.83% (heart, $f \gt 200$ MHz).